The desirability of making glass containers of very light weight without sacrificing the strength of the container has long been known. The container that is most extensively produced in the United States is the 12 ounce beer bottle. As presently manufactured, this bottle weighs about 7 to 71/4 ounces. It is, however, characterized by large and easily measured variations in wall thickness. The areas of minimum thickness must be strong enough that the bottle will withstand all of the known test procedures to insure that it will perform satisfactorily on a capping machine and will not break when subjected to mechanical abuse and the stresses arising from increased internal pressure caused by prolonged exposure of its contents to a hot environment. If the minimum thickness areas are sufficiently strong, it is apparent that any thicker areas represent a waste of material and the energy required to produce it.
Prior art bottles are produced by forming a parison that is only slighter shorter than the height of the blow mold cavity in which the bottle is finally formed. This parison is introduced into a blow mold, permitted to "run" by its own weight and fluidity for a predetermined time and then blown, usually by introducing high pressure air into the interior of the parison to expand it into intimate contact with the interior of the blow mold.
A "short" parison (75% or less of blow mold cavity height) is formed more easily and uniformly on the blank side because of its lower ratio of length to average diameter. A shorter parison can also be transferred to the final blow mold cavity faster and with less distortion in the inventing because the effect of the centrifugal and gravitational forces acting on the parison during this time is accentuated by the distance of the center of gravity of the parison from the point of intersection of the invert arm radius with the center line of the parison. Furthermore, in reducing the weight of a given bottle to the absolute practical minimum there is just not enough mass of glass available to make a parison of the conventional length and design. When the glass gob contacts the blank mold the glass surface cools at the rate of 1700.degree. C. (3100.degree. F.) per second. This means that in 0.1 seconds it loses 170.degree. C. (310.degree. F.) so the short parison with its lesser ratio of surface to mass is necessary to prevent the glass mass from cooling so fast as to cause a structural failure of the material or a stiffness that makes it unworkable.
The short parisons used in my invention may be produced by an improved parison forming method, described in my copending application, Ser. No. 112,845, filed Jan. 17, 1980.
A "short" parison requires so much remelt time and "run" time (because it has more than five times as much distance to "run" and less gravitational force due to its lesser weight) that its operation by conventional means would be uneconomic from a production speed standpoint.
In order that the rate of production of bottles of more uniform wall thickness by equal to or greater than the rate of production of conventional machines, I have proposed that a plurality of blow molds be associated with each blank mold. Such an apparatus should be compatible with the forming stations of a conventional Hartford IS machine because of their widespread use in the industry. Such an apparatus is shown, described and claimed in my copending application, Ser. No. 42,455, filed May 25, 1979.